In recent years, rubber compositions for tire treads containing rubber such as natural rubber as a main component have been proposed in consideration of exhaustion of petroleum resources, reduction in rolling resistance, the environment, and the like (see, for example, Patent Document 1). However, for example, the rubber composition for a (cap) tread with natural rubber (NR) used as a main rubber component is problematically inferior in wet grip performance to a conventional rubber composition for a tread with styrene butadiene rubber (SBR) or the like rubbers. For this reason, epoxidized natural rubber (ENR) has been used for a tire tread in order to improve wet grip performance, and thus the exhaustion of petroleum resources and reduction in CO2 emission have been positively addressed.
Thus, the use of the ENR provides a rubber composition for a (cap) tread with good wet grip performance. However, if the rubber composition is used particularly for high performance (very low-profile) tires, and tires for heavy load vehicles among passenger vehicles, the rubber composition is required to be further improved in abrasion resistance, compared with the conventional rubber composition for a tread containing SBR.
In a rubber composition for a (cap) tread, silica has been used as a reinforcing filler because the fuel economy is compatible with the wet grip performance. However, it is very difficult to favorably disperse silica, especially finely-divided silica, in a rubber component containing a predetermined amount or more of ENR. A silane coupling agent is usually used in the silica-containing rubber composition in terms of silica dispersibility and the like. A mercapto group-containing silane coupling agent is highly reactive and has high performance but has a very short scorch time. Thus, it is very difficult to practically use the mercapto group-containing silane coupling agent in the tire industry.
Meanwhile, rubber compositions containing carbon black, as a reinforcing filler, with a rubber component that is obtained by blending NR which shows excellent tensile strength and tear strength, butadiene rubber (BR) which improves flex crack growth resistance, and the like rubbers have been conventionally used for sidewalls and base treads of tires. However, similarly to the rubber composition for a tread, the fuel economy of the rubber composition for a sidewall or a base tread is recently required.
For this reason, a large proportion of or otherwise all of the carbon black is considered to be replaced with a white filler such as silica. However, the replacement with a white filler often causes reduction in dispersibility compared with the carbon black. Thus, breaking energy (tensile strength×elongation at break) is less likely to be sufficiently secured, and therefore crack resistance and flex crack growth resistance problematically decrease. In consideration of exhaustion of petroleum resources and tightening of CO2 emission regulation, it's not just that the carbon black is replaced; synthetic rubbers such as BR are desirably replaced with rubbers derived from non-petroleum resources such as ENR so as to increase the percentage usage of non-petroleum resources in a tire.
For meeting these demands, Patent Documents 2 and 3 disclose rubber compositions for a sidewall which contain silica with NR and ENR so as to increase the content of non-petroleum resources, and have good flex crack growth resistance and tear strength. However, the crack resistance, flex crack growth resistance, and the like performances of the rubber compositions are still required to be improved particularly in case of use for high-performance tires.    Patent Document 1: JP 2003-64222 A    Patent Document 2: JP 2006-70093 A    Patent Document 3: JP 2007-56205 A